The field of the invention is ion mobility spectrometers. Ion mobility spectrometry (IMS) is an important method for detecting drugs, explosives, VOCs, and chemical warfare agents at ambient pressure. Explosives generally have high electron affinities and drugs and chemical warfare (CW) agents have high proton affinities. When these chemicals enter the reactor of an ion mobility spectrometer (IMS), negative and positive ions of these samples will be preferentially formed. Such a preference allows a high sensitivity of IMS technology in detecting trace explosives, drugs, and CW agents. Some commercial ion mobility spectrometers are available for detecting the above chemicals.
Miniaturization of such instruments provides advantageous applications in the field. However, a typical problem for commercial hand-held IMS is loss of sensitivity. For example, the sensitivity of a desktop size IMS detector now used in airports, is about 1 nanogram for explosives. The sensitivity of a smaller, handheld version, would be reduced more than 100 times. The main reason for the reduced sensitivity is the use of a nickel-63 (Ni63) radioactive source for ionization. Nickel-63 emits electrons with 67 keV kinetic energy. The low stopping power of the high-energy electrons in gases generates less ions in the small volume of the miniature IMS ionization chamber, resulting in the low sensitivity. In addition, a nickel-63 source has potential hazards due to its radioactive nature. An example of an ion-producing device with a nickel-63 radioactive source is disclosed in Turner et al., U.S. Pat. No. 6,225,623, issued May 1, 2001. For general information concerning the principles of ion mobility spectrometry, reference is made to Eiceman, G. A. and Karpas, Z., xe2x80x9cIon Mobility Spectrometry,xe2x80x9d CRC Press, Boca Raton Fla., USA, 1994.
In Taylor et al., U.S. Pat. No. 5,684,300, issued Nov. 4, 1997, and PCT Pub. No. WO 03/11554, published June 10, 1993, pulses with various polarities, amplitudes, and widths are generated by a RF oscillator and are used to produce ions through a corona discharge. Certain features of these pulses are undefined, which tends to limit the performance of this kind of spectrometer. An ion gate is used to control ions entering an ion mobility channel and the electronics require that the device have extra size.
The invention is a method and apparatus for providing a pulsed discharge ionization source particularly designed for miniature ion mobility spectrometers (IMS), but also usable in other analytical instruments. The invention uses a pulse to generate a corona around a tip of non-radioactive (non-doped) material to generate ions from a sample gas and to signal the start of ion motion.
In a further aspect of the invention, the applied potential comprises a pulse component and a dc base voltage component, which reduces the pulse component. This reduces noise and power consumption.
Miniaturized ion mobility spectrometers equipped with the pulsed discharge ionization source of the present invention have the following advantages: (1) high sensitivity because the ions are concentrated in a very small volume, (2) the use of an ion gate and its associated electronics is unnecessary, and (3) a high dynamic range is available because the ionization rate can be adjusted. The present invention provides a method and an apparatus in which ions are generated in a highly confined space and time, which results in high sensitivity for miniature IMS detectors. A processor-based electronic control enables timing of the initial ion motion with the ionization pulse. This provides a device without the need for an ion extract gate for ions entering a drift chamber. This reduces the size of the drift chamber body, the electronics control package, and power consumption. The invention also provides for increased dynamic range by adjusting the pulse height or by adjusting the DC bias.
Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follows. In the description reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however are not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.